1. Field of the Invention.
The present invention relates to high-pressure, low-delivery liquid pumps used in paper machines for waterjet cutting or slitting a continuous traveling fiber material web, such as a paper web.
2. Description of the Related Art.
Waterjet cutters or slitters are used in a paper machine to cut a traveling fiber material web. A moving high-pressure stream of liquid which is used for cutting affords advantages in that the liquid can be ordinary water without additives. The cutting operation is dust-free and does not create dust problems which result in creating wear in machinery, pollution of the air and health hazards. Waterjet cutting is advantageous in that it does not require space-consuming and complex cutting equipment, and the mechanism can be easily operated and controlled for a variety of cutting conditions with variations in speed and thickness of material and other variations which must be encountered in commercial cutting operations.
However, to provide a continuous supply of water in very small quantities at very high pressures such as used in paper web cutting, the reliable operating life of most conventional pumps is severely limited. Pressures in the range of 10,000 psi to 60,000 psi must be available at very small delivery quantities of water and such pumps frequently have an operating life on the order of only 250 hours without requiring shutdown and attention. Further, such pumps require parts with critical tolerances and the moving parts must be carefully and precisely machined. Also, conventional pumps are generally mechanically complex and many are unreliable in applications where the fluid is water and cannot contain rust inhibitors.
U.S. Pat. No. 4,239,448 (Graf), now expired, describes a high-pressure, low delivery liquid pump for use in a paper machine which is simple, effective and reliable. A hydraulic wedge is created between a hydraulic shoe and a rotating inwardly facing surface of a pump shell. A fluid passage having an inlet at the working surface of the shoe transports high-pressure liquid away from the hydraulic wedge to a cutting head of a slitter.
The high-pressure, low delivery liquid pump described by Graf '448 requires a control system to ensure that the liquid within the pump adjacent to the pump shell is kept at a relatively small predetermined level. The fluid passage only draws away a relatively small amount of water under high pressure from the hydraulic wedge. If too much water is introduced into the pump, the hydraulic shoe must "push" the excess water ahead of the shoe as the pump shell rotates. The excess water is not actually immediately used since it is not in the high-pressure region of the hydraulic wedge, and substantially adds to the power requirements to drive the rotation of the pump shell. The necessary increased power in turn decreases the efficiency of the pump and increases the cost to operate the pump. It is therefore necessary to provide a relatively complicated and expensive control system to control the water flow into and operation of the pump.
What is needed in the art is a high-pressure, low delivery liquid pump for use in a paper machine which maintains necessary input power at a minimum by removing excess or overflow water from the pump without the need for a complex and expensive control system.